Membrane switches have at least one contact that is on, or made of, a flexible substrate, i.e. a membrane layer. There is always a second layer against which the membrane layer is pressed, and that second layer can be either static or flexible.
Although it is possible to produce membrane switches that rely upon capacitive, ferrite core, or hall effect, membrane switches typically utilize a direct (Ohmic) contact in which the poles of the switch make transient physical contact. Thus, pressure on the membrane layer physically closes a circuit by contacting one electrical trace to another, and upon release these “poles” separate as the flexing membrane returns to its original position.
The momentary type of action, ready accommodation to visual design feature, low cost, and relatively high reliability, all cooperate to provide applicability in innumerable applications. Among other things, membrane switches are very commonly used in DC logic-level circuits such as those used in computer keyboards, on medical equipment, and in television and other hand-held controllers.
Although membrane switches are fairly reliable, they are known to fail. Failures can occur for any number of reasons, including operator error, moisture, excessive use, manufacturing defects, and so forth. In some applications failure carries an extremely high cost, and it is therefore necessary to utilize some sort of self check that identifies a switch as being defective, or at least allows the switch to continue functioning with a defect. Exemplary applications requiring failsafe operation include pressure sensing floor mats used for security purposes, and operational controls on life support equipment.
As used herein the term failsafe device means that the device can be interrogated at all times to detect a failure. Failsafe capabilities appear to be unknown in membrane switches. Existing membrane technology generally relies upon orthogonally parallel traces upon opposing surfaces. In normal operation the traces do not touch each other, and there is only a single lead from each trace. Such designs are not conducive to failsafe operation because there is no way to test the integrity of the traces. Indeed, Touch-Sensor™ advertises their TouchCell™ switches (which are not membrane switches) as the only “touch technology” switches that are recognized by UL as failsafe switches. (http://www.touch sensor.com/faq.html). The Touch Sensor™ web page, as well as all other patents, applications, web sites, articles and the like referenced herein are incorporated by reference in their entirety. Where a definition or use of a term in a reference, which is incorporated by reference herein, is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
U.S. Pat. No. 5,175,443 to Tabuchi (December 1992) does teach a membrane switch that can detect a false positive (always on) condition. There, a membrane switch has closely disposed duplicate traces. When the switch is operating normally, pressure sufficient to establish a circuit in one of the duplication traces is assumed to concurrently establish a circuit in the close duplicate. If one of the switches becomes defective because one of the traces has peeled away from its base, a logic circuit can detect the failure by comparing the current in the duplicate traces. Unfortunately, devices using to the '443 technology are only able to detect false positive situations—they are unable to detect false negative situations, in which the switch fails to record a proper “on” situation. Such switches are not considered failsafe as the term is used herein because the integrity of apparently viable traces cannot be tested.
Existing membrane circuits are also designed to detect pressure at a given point, on the membrane, or pressure on the membrane at any point. And such circuits merely detect on-off. Membrane circuits are apparently unknown that detect sizes and shapes (i.e., footprints) or weights. Significantly, it is exactly in detection of sizes, shapes and weights that failsafe operation is so critical.
Thus, there is still a need for additional development of failsafe membrane switches, especially for membrane switches that detect sizes, shapes and/or weights